In operating an internal combustion engine, it is necessary to establish various control parameters. Such control parameters may include the quantity of fuel supplied to each cylinder prior to combustion and also the spark advance angle. Until recently, such control parameters were established by mechanical devices responsive to various engine operating parameters. Careful testing of engines shows that optimum values for these control parameters are complex functions of the operating parameters and that these functions cannot be matched by mechanical devices. Modern engine control systems use empirically derived characteristics for the control parameters which are stored as look-up tables in read only memories.
Although these look-up tables provide a much closer match to the optimum characteristics than was achieved by the mechanical devices, they still do not achieve optimum values for the control parameters. There are a number of reasons for this. These reasons include variations between test engines and production engines and also various aging effects which occur during the life of an engine.
In U.S. Pat. No. 4,379,333, incorporated by reference herein, there is described an adaptive control system for controlling the spark advance angle. In this system, small positive and negative perturbations are superimposed on the spark advance angle and the resulting changes in speed are used to determine the differential or slope of engine output with respect to spark advance angle. The slope is used to update the spark advance angles stored in a look-up table.
In the arrangement described in this patent, the perturbations are imposed in a three phase cycle, a positive perturbation being imposed during a first phase, no perturbations being imposed during the next phase, and a negative perturbation being imposed in the last phase. Each phase comprises 40 to 60 engine fires. The engine speed is sensed over a number of engine fires at the end of each phase. The perturbation of the spark advance angle will cause an initial transient response in the engine speed at the beginning of the phase, but, by the end of the phase, a steady speed response is obtained. Thus, by delaying measurement of the engine speed until the end of the phase, the transient response is eliminated. However, this method suffers from the disadvantage that a large number of engine fires is required to make each slope measurement.
There is one further important disadvantage of the arrangement described in this patent. In this patent, during each measurement phase, the perturbations are applied to all the cylinders of the engine. Similarly, there is a single spark advance look-up table for all cylinders. In practice, there are significant variations between the individual cylinders of an engine, and so there are significant differences between the optimum values for the various control parameters between the various cylinders. The case of spark advance angle may be considered by way of illustration. Where a single spark advance value is used for all cylinders, then under particular operating conditions there will be an optimum spark advance angle at which maximum torque output is achieved. Similarly, for each individual cylinder, there is also an optimum spark advance angle for achieving maximum torque output. This angle will not, in general, be identical for each cylinder.